Hum from Crown XLR2000 Power Amp

I developed a bad hum in my recording gear after adding a McOne Passive Monitoring Controller.
SPOILER: the solution to my hum is highlighted at the bottom of the page for others with hums and no time to read the rest.
The McOne is so I can ABC my Sonos Player 5 Gen2‘s, SN-10 reference monitors and the transistor radio. As well as wanting to play into these from LogicX, I wanted to play computer sound without having to select the Duet interface, and my record player.
So, with three inputs and three outputs, I had a hum on the reference monitor going through the CrownXLS2000 power amp. This hum stopped on pressing the McOne’s mute switch, but returned even on mute when the other two outputs were selected. So the culprit might be the McOne, as well as the power amp, cables etc. Not a great prospect…
I really didn’t want it to be the McOne. I’d bought it from Gear4Music, who as they messed up the order, gave me the even more expensive stepped version at a huge discount. It is so well built, clean and high quality; with a great transparency from which the Sonos – and even my low budget monitors – benefit greatly. (Let’s say I was pretty pleased with this piece of kit.) I was also feeling the after effects of an all-night party that ended early morning with a 6 mile walk through central London, coffee and croissants at a Pret on Oxford Street as it opened (0530) then an early morning coach ride back to Oxford as the hotel room was so bloody dreadful, noisy with a ridiculous bright light just outside the window and ill-fitting curtains…. I mention the Pret as a tip for post central London, pre-drive gig refuelling; the croissants were just out of the oven and utterly perfect.
So having ensured there was no earth loop from other components (by ensuring all were connected via the one power strip from the one ring main, disconnecting all, then adding back one by one), I started with the Crown, and changed all the cables. When I disconnected its earth lead at the power plug (creating a so-called “cheat plug”), the hum lost some of its lower frequencies (but none of its unacceptability). I immediately re-connected the earth lead.Warning: Just don’t use cheat plugs chaps!! They are dangerous. You can forget you’ve done them, hence my changing it back immediately. I also didn’t use the Crown-supplied lead, but used another kettle lead, plus I left the green earth wire sticking out of the plug casing in a very obviously dangerous-looking manner.
I was very disinclined to try a noise suppressor. They also suppress frequencies, and don’t address the actual problem.
So with all brand new cables and no change, it was time to dance with tambourines – as IT department people say in Russia when they haven’t a clue what’s wrong. The McOne requires TRS balanced ins and outs. Disconnecting the jack outs to XLR ins on the Crown stopped the hum. I’d previously used mono jack to RCA cables (from the Duet to the RCA inputs on the Crown). So I tried using this cable, from the McOne to the Crown, and bingo, the problem was solved!

SOLUTION: Using unbalanced (mono with single rings on the 1/4 jacks) ins to the RCA ins on the Crown. So, with your hum problem, you could try using unbalanced cables – or even the RCA inputs as opposed to the XLRs. I have no idea why this might work, but hope it does for you!